1. Field of the Invention
This invention relates generally to a method of driving a dielectric barrier discharge device and, more particularly, to a method of providing a pulsed power signal to drive a dielectric barrier discharge device, where the pulsed signal has periodic groups of opposite-polarity pulses so as to reduce wasted energy in the device.
2. Discussion of the Related Art
It is known in the art to use plasma discharges to convert various pollutants, such as VOCs, NOx, SOx, particulates, etc., within gaseous streams from, for example, engines, factory smokestacks, power plants, incinerators, etc., to harmless by-products to reduce the environmental impact of the pollutants. A plasma discharge can be generated in different ways, such as by electron beams, corona discharges and dielectric barrier discharges, all well known in the art. Each of these types of plasmas generate a discharge that includes electrons, positive ions and an assortment of chemical radicals that act to break down the pollutants into the harmless by-products. The use of plasma discharge conversion is receiving greater attention and consideration for eliminating pollutants in vehicle exhaust. One particular area of concern is the elimination of nitrogen oxides (NOx) from diesel exhaust. The research so far has indicated that dielectric barrier discharge devices will be the most practical to be incorporated into a vehicle exhaust system.
FIG. 1 shows a schematic view of a simplified dielectric barrier discharge device 10. The discharge device 10 includes a first dielectric plate 12 and a second dielectric plate 14 made of a suitable dielectric material, such as alumina, quartz, or any suitable high dielectric strength insulator. A first electrode 16 is formed on a surface of the first dielectric plate 12 and a second electrode 18 is formed on a surface of the second dielectric plate 14, so that the dielectric plates 12 and 14 are both positioned between the electrodes 16 and 18, and an air gap region 20 is formed between the dielectric plates 12 and 14. The electrodes 16 and 18 can be formed by any suitable process, such as by sputtering a thin layer of gold on the surface of the plates 12 and 14. A power generator 22 provides a potential to the electrodes 16 and 18 to operate a high strength electric field across the gap region 20 that creates the plasma discharge. The gas, such as a vehicle exhaust gas, to be converted is caused to flow through the gap region 20 between the plates 12 and 14 in the electric field. Of course, a working dielectric barrier discharge device would include additional components as would be apparent to those skilled in the art. However, the depiction of the device 10 as shown here is suitable for purposes of the present invention.
If the potential applied to the electrodes 16 and 18 creates a large enough electric field, a dielectric breakdown occurs in the gas within the gap region 20 that creates a discharge. The dielectric breakdown occurs as a result of free electrons in the gas being accelerated towards the plate 12 or 14 associated with the positive potential electrode 16 or 18, and then colliding with gas molecules to cause the gas molecules to ionize and create more electrons and positive ions having high energy. The greater the electric field, the higher the energy of the electrons and the positive ions. The potential applied to the electrodes 16 and 18 is optimized to create the desired amount of ionization for a particular system. This ionization process creates a streamer trail of ionization consisting of electrons, positive ions, and an assortment of chemical radicals that propagates across the gap region 20 towards the higher potential (positive) dielectric plate 12 or 14.
When the streamer trail reaches the surface of the positive dielectric plate 12 or 14, the electrons which surround the head of the streamer trail are trapped on the surface of the dielectric plate 12 or 14. As a result, the electric field is locally reduced at this location on plate 12 or 14 and the streamer trail is extinguished. This event is referred to as a micro-discharge that lasts for a very short time (5-20 nanoseconds) and transfers only a small amount of energy. Since the electrodes 16 and 18 remain charged with a high voltage, the macroscopic electric field remains. Thus, other micro-discharges occur at other locations in the gap region 20. If the dielectric plates 12 and 14 were not positioned between the electrodes 16 and 18, then the electrons in the streamer trail would conduct into the positive electrode 16 or 18, creating an ionized column having low resistance, which would cause all of the energy within the gap region 20 to dump through that column. The barrier process generates a large number of extremely short pulses that cause the volume between the plates 12 and 14 to fill with a series of micro-discharges. The relevant literature provides a more detailed discussion of the dielectric breakdown process.
As this process continues, a net charge build up occurs on the surface of the positive dielectric plate 12 or 14. If the voltage provided by the power generator 22 was constant (DC), where one of the electrodes 16 or 18 was always positive, eventually the charge would accumulate on the positive plate 12 or 14 to a high enough value to reduce the electrical field across the entire gap region 20, and discharges in the gap region 20 would cease or an arc-over failure would occur. To avoid this catastrophic occurrence, it is known in the art to energize the electrodes 16 and 18 with an AC voltage signal or periodic pulses of the same polarity. When using an AC signal to drive the electrodes 16 and 18, the AC frequency is optimized to be high enough to avoid an excessive charge build-up on the positive dielectric plate 12 and 14, but low enough to limit the power input to the gap region 20 to prevent damage to the plates 12 and 14 or unnecessary, wasted power. For pulsed inputs, the pulse duration and period is selected to avoid a charge build-up.
U.S. Pat. No. 5,603,893 issued to Gundersen et al., Feb. 18, 1997, discloses an electrostatic precipitator that operates as a discharge device similar to that discussed above. The electrostatic precipitator generates a corona discharge that creates both beam-like electrons and thermal-like electrons to increase the effective plasma volume. A pulse modulator generates a pulsed output that is applied to saw tooth electrodes in the discharge device at a particular pulse width, repetition rate and rise time to create these types of electrons. The pulsed output appears to only include pulses of the same polarity.
The formation and extinguishment of the streamer trails created during the dielectric breakdown, as discussed above, cause electrons to accumulate on the surface of the positive plate 12 or 14 that is opposite to the electrode 16 or 18. This accumulation of charge creates a capacitance formed by the layer of electrons on the surface, the dielectric plate 12 or 14, and the electrode 16 or 18 on the opposite surface. When the pulse is turned off or the AC signal changes sign, the accumulated charge on the surface of the dielectric plate 12 or 14 will leak away due to parasitic resistance discharging the capacitance, and will result in adding entropy to the system. When this charge stored on the surface of the dielectric plate 12 or 14 leaks off between pulses applied to the electrode 16 or 18, as occurs for discharge devices using typical pulsed power supply inputs, the power stored by this capacitance is wasted.
For example, the energy E stored in this capacitance C is given by: EQU E=CV.sup.2
where V is the voltage appearing across the dielectric barrier. If the gap width is 1 mm, the plates are 25 mm in width and 95 mm in length, and the plates are charged to a voltage of 2 kV at a frequency of 1 kHz, the capacitance C, stored energy E, and power P are, C=84 pF, E=0.16 mJ, and P=0.16 W. A typical system as just described, only deposits 0.5-2 watts of power in the gas, so that the loss due to charge leakage between pulses is not negligible.
What is needed is a dielectric barrier discharge device that uses at least some of the power that is generally wasted by the leak-off of the charge build-up on the dielectric plate opposite to the electrode, so as to increase the conversion of pollutants in the gas stream to harmless by-products. It is therefore an object of the present invention to provide such a dielectric barrier discharge device.